MEMS pressure sensor using capacitive technique

ABSTRACT

A micro-electro-mechanical system (MEMS) pressure sensor includes a silicon spacer defining an opening, a silicon membrane layer mounted above the spacer, a silicon sensor layer mounted above the silicon membrane layer, and a capacitance sensing circuit. The silicon membrane layer forms a diaphragm opposite of the spacer opening, and a stationary perimeter around the diaphragm and opposite the spacer. The silicon sensor layer includes an electrode located above the diaphragm of the silicon membrane layer. The capacitance sensing circuit is coupled to the electrode and the silicon membrane layer. The electrode and the silicon membrane layer move in response to a pressure applied to the diaphragm. The movement of the silicon membrane layer causes it to deform, thereby changing the capacitance between the electrode and the silicon membrane layer by an amount proportional to the change in the pressure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to application Ser. No. 12/034,667, filedFeb. 21, 2008, which is incorporated herein by reference.

FIELD OF INVENTION

This invention relates a pressure sensor, and more particular to amicro-electro-mechanical system (MEMS) pressure sensor.

DESCRIPTION OF RELATED ART

U.S. Pat. No. 7,187,185 discloses force sensors that use area-changecapacitive sensing techniques to determine the magnitude of the force.These sensors are made using conventional machining processes.

In one device, a first vertical electrode is mounted on a top layer anda second vertical electrode is mounted on a bottom layer. The top andthe bottom layers are separated by an inert gas. When a force isasserted, it changes the overlap area and therefore the capacitancebetween the vertical electrodes. The change in the capacitance can becorrelated to the magnitude of the force applied to the device.

In another device, an inner conductive surface in the form of a metaltube is inserted into a middle conductive surface in the form of ahollow tube. A spacer may hold the inner conductive surface inside themiddle conductive surface so they are separated by a gap. Like theearlier device, when a force is asserted, it changes the overlap areaand therefore the capacitance between the conductive surfaces. Thechange in the capacitance can be correlated to the magnitude of theforce applied to the device.

SUMMARY

In one embodiment of the invention, a micro-electro-mechanical system(MEMS) pressure sensor includes a silicon spacer, a silicon membranelayer mounted above the spacer, and a silicon sensor layer mounted abovethe silicon membrane layer. The silicon spacer defines an opening to thesilicon membrane layer. A first portion of the silicon membrane layeropposite of the spacer opening forms a diaphragm while a second portionof the silicon membrane layer opposite of the spacer forms a stationaryperimeter around the diaphragm.

The silicon sensor layer includes an electrode located above thediaphragm. A capacitance sensing circuit is coupled to the electrode andthe silicon membrane layer. The electrode and the silicon membrane layermove in response to a pressure applied to the diaphragm. The movement ofthe silicon membrane layer causes it to deform, thereby changing the gapbetween opposing surfaces of the electrode and the silicon membranelayer. This in turn changes the capacitance between the electrode andthe silicon membrane layer by an amount proportional to the change inthe pressure. An increase in the capacitance indicates the electrode andthe silicon membrane layer are translating in one direction, and viceversa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a side view of a MEMS pressure sensor in oneembodiment of the invention.

FIG. 2 illustrates a top view of the MEMS pressure sensor of FIGS. 1Aand 1B in one embodiment of the invention.

FIG. 3 illustrates a side view of the MEMS pressure sensor of FIG. 1under pressure in one embodiment of the invention.

FIGS. 4A, 4B, 4C, and 4D illustrate a process for making the MEMSpressure sensor of FIG. 1 in one embodiment of the invention.

FIG. 5 illustrates a top view of the MEME pressure sensor of FIG. 1 inanother embodiment of the invention.

FIG. 6 illustrates a side view of a MEMS pressure sensor in oneembodiment of the invention.

FIG. 7 illustrates a side view of a MEMS pressure sensor in oneembodiment of the invention.

Use of the same reference numbers in different figures indicates similaror identical elements.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 illustrate a MEMS pressure sensor 100 in one embodiment ofthe invention. Sensor 100 is formed using conventional semiconductormanufacturing processes.

Sensor 100 includes a silicon spacer 102 that defines an opening 104. Asilicon membrane layer 406 is mounted above spacer 102. A portion ofmembrane layer 406 exposed by opening 104 forms a diaphragm 108 and aportion mounted above spacer 102 forms a stationary perimeter 110 aroundthe diaphragm. In one embodiment, opening 104 is a round opening anddiaphragm 108 is therefore round.

A first oxide layer 404 has a portion 114 between spacer 102 andstationary perimeter 110. As will be describe later, oxide layer 404acts as an etch stop in the process for making opening 104 in spacer102.

A silicon sensor layer 410 is mounted above membrane layer 406. Sensorlayer 410 includes a movable electrode 118 and a stationary electrode120 around the movable electrode. Movable electrode 118 is locatedsubstantially above diaphragm 108 while stationary electrode 120 islocated substantially above stationary perimeter 110. Movable electrode118 and stationary electrode 120 are separated by a gap 122.

A second oxide layer 408 has a portion 126 between movable electrode 118and diaphragm 108, and a portion 128 between stationary electrode 120and stationary perimeter 110. Oxide layer 408 acts as an insulatinglayer to prevent electrodes 118 and 120 from being shorted.

FIG. 2 illustrates one embodiment where movable electrode 118 is a diskand stationary electrode 120 has a round inner perimeter that matchesthe shape of the movable electrode. As a result of oxide etch andrelease of movable electrode 118 and stationary electrode 120 from oxidelayer 408, holes 202 (only two are labeled for clarity) are formed onthe outer circumference of movable electrode 118 and the innercircumference of stationary electrode 120. Note that the phantom linesshown indicate the perimeter of oxide portions 126 and 128 from oxidelayer 408 (FIG. 1).

Referring back to FIG. 1, the capacitance between the electrodes can besensed by a capacitance sensing circuit 132 when a voltage is suppliedto electrodes 118 and 120. The value of the capacitance depends on theoverlap area and the gap between the sidewalls of electrodes 118 and 120as follows:C=ε ₀ A/d,where C is the capacitance, ε₀ is the permittivity of free space, A isthe overlap area between the electrodes, and d is the gap distance.

Referring to FIG. 3, a pressure difference between the opposing sides ofdiaphragm 108 (simply illustrated as pressure 130 asserted on thediaphragm) causes the diaphragm and movable electrode 118 to translate.As the pressure is evenly distributed across diaphragm 108, thetranslation of the diaphragm and movable electrode 118 are substantiallyvertical so gap 122 between the movable electrode and stationaryelectrode 120 remain substantially constant.

This vertically translation of movable electrode 118 changes the overlaparea (symbolically illustrated by a distance 302) between sidewalls ofmovable electrode 118 and stationary electrode 120. The change in theoverlap area changes the capacitance between electrodes 118 and 120. Thechange in the capacitance is linearly proportional to the change inpressure 130. The exact relationship can be mathematically orempirically determined to find the change in pressure 130.

FIGS. 4A, 4B, 4C, and 4D illustrate a method for making sensor 100 usingconvention semiconductor manufacturing processes in one embodiment ofthe invention. In the first step illustrated in FIG. 4A, a doublesilicon-on-insulator substrate 400 is provided. Substrate 400 includes afirst silicon layer 402, a first oxide layer 404 above silicon layer402, a second silicon layer 406 above oxide layer 404, a second oxidelayer 408 above silicon layer 406, and a third silicon 410 above oxidelayer 408. As used here, a silicon layer refers to any material withsilicon, including single crystal silicon, polysilicon, and siliconcarbide. Silicon layer 406 can be selected to be stiff or soft dependingon the sensitivity to pressure desired for sensor 100.

In the second step illustrated in FIG. 4B, silicon layer 402 is etcheddown to oxide layer 404 to form spacer 102 with opening 104. In thisstep, oxide layer 404 acts as an etch stop.

In the third step illustrated in FIG. 4C, silicon layer 410 is etched toform electrodes 118 and 120. Also in this step, holes 202 (FIG. 2) areetched in the outer perimeter of electrode 118 and the inner perimeterof 120 in preparation for the oxide etch and release in the next step.

In the fourth step illustrated in FIG. 4D, an oxide etch is performed toremove portions of oxide layer 404 and 408 exposed by silicon layers 402and 410. Specifically, this releases electrode 118 and diaphragm 108from the remainder of the structure so they can translate verticallyunder a change in pressure.

FIG. 5 illustrates a variation of MEMS pressure sensor 100 (hereafter“sensor 500”) in one embodiment of the invention. Sensor 500 isessentially the same as sensor 100 except that a movable electrode 118Anow has spokes 501 with teeth 502 that are interdigitated with teeth 504(only one of each is labeled for clarity) from a stationary electrode120B. Teeth 502 and 504 increase the overlap area between electrodes118A and 120A at any vertical displacement in order to make sensor 500more sensitive to pressure changes. In addition, movable electrode 118has spokes 506 that are coupled by springs 508 to stationary pads 510(only one of each is labeled for clarity) insulated from stationaryelectrode 120A. This allows wire bonds to be formed two or morestationary pads 510 for coupling to voltage sources. Whileinterdigitated teeth are shown, other complementary features may be usedto increase the overlap area between electrodes 118A and 120A.

FIG. 6 illustrates a variation of MEMS pressure sensor 100 (hereafter“sensor 600”) in one embodiment of the invention. Sensor 600 isessentially the same as sensor 100 except a channel 602 on the topsurface of membrane layer 406 is formed below gap 122. Channel 602allows membrane layer 406 to be more flexible so it can translate undersmaller pressure differences. In the fabrication of sensor 600, spacer102, oxide layer 404, and membrane layer 406 can be made from a singleSOI substrate. Channel 602 is formed by etching the top surface ofmembrane layer 406, and then forming oxide layer 408 and bonding sensorlayer 410 above of oxide layer 408.

FIG. 7 illustrates a variation of MEMS pressure sensor 100 (hereafter“sensor 700”) in one embodiment of the invention. Sensor 700 isessentially the same as sensor 100 except the capacitance betweenelectrode 118 and membrane layer 406 is sensed by a capacitance sensingcircuit 732 when a voltage is supplied to electrode 118 and membranelayer 406. Sensor 700 may or may not include capacitance sensing circuit132.

As described above in reference to FIG. 3, a pressure difference betweenthe opposing sides of diaphragm 108 of membrane layer 406 causes thediaphragm 108 and movable electrode 118 to translate. As membrane layer406 deforms in the translation, the gap between the opposing surfaces ofelectrode 118 and membrane layer 406 changes. The change in the gapchanges the capacitance between electrode 118 and membrane layer 406.The change in the capacitance is nonlinearly proportional to the changein pressure 130. The exact relationship can be mathematically orempirically determined to find the change in pressure 130.

The change in the capacitance between electrode 118 and membrane layer406 measured by capacitance sensing circuit 732 indicates the directionof movement. The capacitance between electrode 118 and membrane layer406 decreases when membrane layer 406 translates upward because the gapbetween the opposing surfaces of electrode 118 and membrane layer 406increases. Conversely, the capacitance increases when membrane layer 406translates downward because the gap between the opposing surfaces ofelectrode 118 and membrane layer decreases. In contrast, the change inthe capacitance between electrodes 118 and 120 measured by capacitancesensing circuit 132 does not indicate the direction of movement. Thecapacitance between electrodes 118 and 120 always decreases regardlessif electrode 118 is translated up or down because the overlap areabetween electrodes 118 and 120 always decreases.

Various other adaptations and combinations of features of theembodiments disclosed are within the scope of the invention. Instead ofusing single or double SOI substrates to form the sensors, multiplesilicon substrate can be bonded together to form the sensors where theoxide layer is grown or deposited on the substrates. Furthermore,although the movable electrode is described as being enclosed by thestationary electrode, the design can be modified where the movableelectrode encloses the stationary electrode. Numerous embodiments areencompassed by the following claims.

1. A micro-electro-mechanical system (MEMS) pressure sensor, comprising:a silicon spacer defining an opening; a silicon membrane layer mountedabove the spacer to form a diaphragm opposite of the opening and astationary perimeter around the diaphragm; a silicon sensor layermounted above the silicon membrane layer, the silicon sensor layercomprising an electrode above the diaphragm; and a capacitance sensingcircuit coupled to the electrode and the silicon membrane layer to sensea capacitance based on a gap between opposing surfaces of the electrodeand the diaphragm; wherein the electrode and the diaphragm move inunison in response to a pressure applied to the diaphragm.
 2. The sensorof claim 1, wherein the capacitance is proportional to the pressure. 3.The sensor of claim 1, wherein the capacitance increases when theelectrode and the silicon membrane layer move in a first direction, andthe capacitance decreases when the electrode and the silicon membranelayer move in a second direction opposite the first direction.
 4. Thesensor of claim 1, further comprising: a first oxide layer between thestationary perimeter of the silicon membrane layer and the spacer; and asecond oxide layer comprising a portion between the electrode and thediaphragm of the silicon membrane layer, the portion bonding theelectrode and the diaphragm, the portion being set back from a perimeterof the electrode.
 5. The sensor of claim 4, wherein the spacer, thefirst oxide layer, the silicon membrane layer, the second oxide layer,and the silicon sensor layer comprise a double silicon-on-insulatorsubstrate.
 6. The sensor of claim 1, wherein the silicon membrane layeris selected from the group consisting of single crystal silicon,polysilicon, and silicon carbide.
 7. The sensor of claim 1, wherein thesilicon membrane layer comprises a channel around a perimeter of thediaphragm.
 8. The sensor of claim 1, wherein the silicon sensor layerfurther comprises an other electrode above the stationary perimeter, theelectrode and the other electrode being separated by another gap.
 9. Thesensor of claim 8, further comprising an other capacitance sensingcircuit coupled to the electrode and the other electrode to sense another capacitance based on an overlap area between opposing surfaces ofthe electrode and the other electrode, the other capacitance beingproportional to the pressure.
 10. The sensor of claim 8, wherein theother electrode surrounds the electrode.
 11. The sensor of claim 8,wherein the electrode is a disk and the other electrode has a circularinner perimeter.
 12. The sensor of claim 8, wherein the electrode is adisk with a plurality of spokes having a first plurality of teeth andthe other electrode has a second plurality of teeth interdigitated withthe first plurality of teeth.
 13. The sensor of claim 12, wherein theelectrode further comprises another plurality of spokes coupled bysprings to stationary pads, and the stationary pads are coupled tovoltage sources.